Photolysis. Wolff rearrangement

Diazo compounds undergo substitution, addition, reduction, and cleavage reactions and the diazo group is retained with the final products. However, the diazo group is eliminated in the case of Wolff rearrangement, photolysis, and Rh-catalyzed C-H insertion reactions. 

The photolysis of o-quinone diazides was carefully investigated by Stis in 1944, many years before the development of photoresists. Scheme 10-102 shows the photolysis sequence for the diazoquinone 10.75 formed in the diazotization of 2-amino-1-naphthol. The product of the photolytic step is a ketocarbene (10.76), which undergoes a Wolff rearrangement to a ketene (10.77). In the presence of water in-dene-3-carboxylic acid (10.78) is formed this compound is highly soluble in water and can be removed in the development step. The mechanism given in Scheme 10-102 was not postulated as such by Stis, because in 1944 ketocarbenes were unknown (for a mechanistic discussion of such Wolff rearrangements see review by Zollinger, 1995, Sec. 8.6, and Andraos et al., 1994). 

Carboalkoxymethylenes, like acylmethylenes, undergo rearrangement to ketenes as well as the olefin addition and C—H insertion reactions characteristic of methylenes.<37> Thus the photolysis of ethyl diazoacetate in olefinic solvents leads to substantial yields of products, which can be rationalized in terms of a Wolff rearrangement of the carboethoxymethylene followed by cycloaddition of the resulting ethoxyketene to the olefin ... 

Thermolysis of 58a in butanol affords, together with 17% of 60a (R = C4H9) which evidences the intermediacy of the thiophosphene 59 a, a variety of partly atypical products which seriously impede the desired rearrangement38. Photolysis of 58b in methanol is also found to give only 18 % 1,2-P/C shift to form the heterocumulene 59b, from which the thiophosphinic rater 60b (R = CH3) results 39). As already mentioned in connection with the photolysis of diazo compounds of type 36 (see Sect. 2.2), Wolff rearrangement (9%) and O/H insertion (6%) once again compete with thiophosphinic ester formation. Moreover, solvolysis of the P(S)/C(N2) bond 391 prevents a greater contribution of carbene products to the overall yield. 

The sensitized reaction yields the cyclopropanes 67 as the only product, whereas direct photolysis results in a Wolff-rearrangement yielding the cyclo-pentaneester 77 AWolff-rearrangement is only given by the singlet carbene... 

In contrast to 2-alkylarylcarbenes, triplet carbonyl carbenes do not abstract H from 5- or e-CH bonds. Photolysis of diazo compounds (7) in methanol gave products due to Wolff rearrangement (8) and 0-H insertion (9). Sensitized photolysis led, in addition, to the H-abstraction product (10). Analysis of the results indicated that a large proportion of the insertion product (9) arises from the excited diazo compound and that spin inversion of the triplet carbene is faster than H-abstraction from the solvent. Intersystem crossing to the singlet state is a major reaction of all triplet carbonyl carbenes that are not rapidly scavenged intramolecularly. 

Figure 18. Diazonaphthoquinone-novolac resist. The novolac (Novolak) matrix resin is prepared by acid catalyzed copolymerization of cresol and formaldehyde. The base insoluble sensitizer, a diazohaphthoquinone, undergoes photolysis to produce a carbene which then undergoes Wolff rearrangement to form a ketene. The ketene adds water which is present in, the film, to form a base soluble, indenecarboxylic acid photoproduct.

During the photolysis of a-diazophosphonium salts, the skeletons of the reactants are preserved. Excluding a plausable Wolff rearrangement, only the O/H insertion product of the carbene with the solvents can be obtained. Diazophosphonium tetrafluoroborate (6), formed by UV irradiation in methanol under a nitrogen atmosphere and subsequent anion exchange with sodium tetraphenylborate, yields the stable (2-methoxy-2-oxopropyl)-triphenylphosphonium tetraphenylborate (7) (equation 4)31. 

The loss of CO, S, SO, S02 and N2 by thermolysis or photolysis has been used to make three- and four-membered rings for example, thiiranes (67) are obtained from (66) (CHEC 5.06.4.4). A2-1,2,3-Triazolines give aziridines and Wolff rearrangement of (68) gives (69). 

Carbene 132 is implicated in the photolysis of 1 since the observed289 photodimerization to 9,10-dihydrophenanthrene and -anthracene is best explained by head-to-head and head-to-tail coupling of this species. Moreover, the fact that allene 134 is isolated289,290 as the major product from irradiation of diesters 31 (equation 35) is fully consistent with a photo-Wolff rearrangement of the carbene. The minor product here involves cyclization... 

Scheme I. Photolysis of a diazonaphthoquinone (DNQ) positive-resist sensitizer. The reaction leads to a carbene (1), which undergoes a Wolff rearrangement to give a ketene (2). Finally, this ketene can react with water present in the resin to give an indenecarboxylic acid (ICA) (3).

To synthesize the very strained tricyclic system of 4 (Fig. 3.12), a photochemical Wolff rearrangement was chosen when MM predicted that the skeleton of 4 should be about 109 kJ mol-1 less stable than that of the available 5. Photolysis of the diazoketone 6 gave a high-energy carbene which lay above the carbon skeleton of... 

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